I. Field of the Invention
The present invention relates to a wireless communications system. In particular, the present invention describes a closed loop power control system.
II. Description of the Related Art
The invention described in this application is discussed in the context of a code division multiple access (CDMA) communications system. However, one skilled in the art will recognize that the invention is adaptable for use in all types of wireless communication systems, especially high-rate systems where data transmission may result in a lack of sufficient transmission power to ensure reliable transmission of all information.
1. CDMA Modulation Techniques
The use of code division multiple access (CDMA) modulation techniques is one of several techniques for facilitating communications in large-scale wireless communication systems. Other multiple access communication system techniques, such as time division multiple access (TDMA) and frequency division multiple access (FDMA) are well known in the art. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled “SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS,” and in U.S. Pat. No. 5,103,459, entitled “SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM,” both of which are assigned to the assignee of the present invention and incorporated by reference herein.
As wireless communication has become more common, there has been an increasing demand for wireless communications systems that can transmit digital information at high-rates. One method for sending high-rate data from a remote station to a base station is to allow the remote station to send the data using the spread spectrum techniques of CDMA. This method allows the remote station to transmit its information using a small set of orthogonal channels, and is described in detail in U.S. patent application Ser. No. 08/886,604, entitled “HIGH DATA RATE CDMA WIRELESS COMMUNICATION SYSTEM,” now U.S. Pat. No. 6,396,804 issued May 28, 2002, assigned to the assignee of the present invention and incorporated by reference herein.
However, as more customers use wireless communication systems, control of the power or energy used to transmit a communication signal becomes even more important. If a communication signal is transmitted at too high a power level, the transmission may interfere with other transmissions. If it is sent at too low a power level, the information contained in the signal might not be received with sufficient reliability. Further, transmitting a signal at the lowest possible power is desirable because, among other things, transmitter battery power in a mobile station can be saved.
2. Power Control
Generally, current “fast” power control techniques used in closed loop wireless communication systems consist of an inner loop and an outer loop. A known and useful method and apparatus for controlling transmission power in this fashion is disclosed in U.S. Pat. No. 5,056,109, entitled “METHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR MOBILE TELEPHONE SYSTEM,” assigned to the assignee of the present invention and incorporated by reference herein. Another technique, used to control the maximum transmission power by gating, capping, or ignoring a power control command, is disclosed in U.S. patent application Ser. No. 09/239,454, entitled “METHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A POTENTIALLY TRANSMISSION GATED OR CAPPED COMMUNICATION SYSTEM,” filed Jan. 28, 1999, now U.S. Pat. No. 6,373,823, issued Apr. 16, 2002, assigned to the assignee of the present invention, and incorporated by reference herein.
As shown in FIG. 1(a), the inner loop controls transmission power by tracking a desired or target signal-to-interference ratio (SIR). A target SIR is selected, and then the estimated SIR for each transmission received by a respective station is compared to the target SIR. If the received transmission is below or above the target SIR, then a responsive up or down (up/down) power control command is sent back to the transmitter to adjust the transmission power as shown in FIG. 1(b). Because these power increases are usually done in increments of ±0.5 dB, the power increases and decreases only approximate the ideal transmit power, as shown in FIG. 1(c).
The outer loop sets the target SIR that is used by the inner loop. Typically, an outer loop employs an algorithm that increases substantially the target SIR if the quality of a decoded received signal degrades below an acceptable level, usually triggered by a frame error detection or an unacceptably high frame error rate (FER). However, as discussed below, when a selective deep fade of a transmitted signal occurs, or when a transmitter cannot increase its transmission power, increasing the target SIR by any amount will not be an appropriate response.
A selective deep fade is a deep fade condition that is short in duration. As shown in FIG. 2(a)—where line 202 represents the strength of an ideal received signal and t represents time—when the strength of the received signal 204 abruptly decreases, thereby causing the quality of the received signal to fall outside an acceptable range 206, a selective deep fade condition has occurred. In current power control systems, when this selective deep fade condition occurs—causing the quality the received signal 204 to degrade—the target SIR will be increased. But due to the temporary nature of the selective deep fade condition, increasing the target SIR may not be an appropriate power control command response, because the cause of the reduced signal reliability is not due to an inappropriately set target SIR.
Another limitation of current power control techniques is the inability to track sudden severe changes in the propagation path of the transmitted signal whereby large increases in signal transmission power are needed for the signal to be received reliably. This problem, referred to herein as a slope overload problem, is illustrated in FIG. 2(b). As shown, when power variation requirements in the signal to be tracked are large compared to the update frequency and “step” size of the actual power changes, quantization of the signal cannot follow. Although the target SIR may be increased or decreased in steps as required, this may not be a correct response, and even if it is, the subsequent change in transmission power will not always be adequate. Current systems generally limit power adjustments to approximately ±0.5 dB (regardless of the amount of degeneration in the propagation path.) The slope overload condition illustrated in FIG. 2(b), shows that when the ideal power requirement P rises quickly, the stepped power increases Pt of current systems cannot keep up.
Yet another limitation of currently known systems is that the inner loop continues to make power increase requests even when the communication system cannot fulfill the request. This power-ceiling problem occurs when the transmitter is incapable of increasing transmission power. For example, the transmitter batteries may be low, or the transmitter's amplifier may be saturated if more power is used. In these cases, repeated “up” power requests may be made by the inner loop power control that cannot be fulfilled. During this period when the transmission energy is capped, the target SIR may be needlessly increased by the outer loop because the signal is received with inadequate reliability.
What is needed is a power control technique that uses a “smarter” outer loop power control. The outer loop power control should be able to detect events that prohibit the transmission power of a signal from being increased or decreased. The outer loop should also be able to detect selective deep fading, a slope overload condition, or system symptoms that indicate that the closed loop power control is not correctly responding to a signal quality problem. Ideally, the outer loop would also be able increase or decrease of power proportional to the detected problem be initiated.